Printed electronic circuits are commonly produced by bonding metal foil to a substrate, usually a synthetic polymer, to form a laminated structure, followed by etching to form the desired circuit. Copper foil is generally used for making such printed circuits, and the copper foil may be produced by rolling or by electrodeposition, although the latter is preferred.
In order to enhance the bonding strength of the foil to the substrate, electrodeposited copper foil is typically produced with a matte surface on one side of the foil which has an increased surface area compared to a smooth surface. To further increase the surface area, and hence the bonding strength, the matte surface often is subjected to further electrochemical treatment to electrolytically form a dendritic copper layer on the matte surface. Also, a two-step electrochemical copper pre-treatment, as disclosed in U.S. Pat. No. Re. 30,180, issued to Wolski and Yates, may be used to further improve the bond strength of the copper foil.
Another problem faced in the manufacture of copper foil is staining or oxidation of the surface of the foil which may occur between the time of manufacture and the time of use by the purchaser. Such staining or oxidation, which may occur during exposure to the atmosphere or as a result of other conditions, is especially undesirable when the foil is intended for use in printed circuit applications, which require very high quality copper foil. Various electrolytic methods have been used for stain-proofing copper foil, for example, the electrolytic application to the foil's surface of hexavalent chromium anions as disclosed in U.S. Pat. No. 3,853,716 issued to Yates and Wolski. While such stain-proofing methods improve the resistance of the foil to staining and corrosion, they have not been found entirely satisfactory as the quality standards for printed circuit foil have been raised. Consequently, other methods have been employed in attempting to improve the resistance to staining and corrosion of the foil.
One such method used is a physical coating method for applying a thin coating of a paint to the surface of the foil, for example, by brushing or spraying. Such methods, however, have been found to be relatively costly, and to present environmental problems. Further, physical coating methods do not provide a completely satisfactory product, in that it is difficult to obtain an even distribution of the paint on the foil, pin-holes tend to occur in the coating and air voids form between the coating and the roughened surface of the foil.
The electrodeposition method has been used to apply paint to fabricated metal parts, such as automobile chassis, furniture and appliance cabinets and the like. Also, U.S. Pat. No. 3,850,773 discloses a continuous electrodeposition method for coating electrical conductors, such as wire and metal foil, with polyamide enamel. This method, however, has not been used prior to the present invention to coat printed-circuit grade copper foil in a continuous electrolytic process, even though a commercially feasible process of this type is highly desirable. Further, until the present invention there has not been developed a commercial electrodeposition process of the above type capable of providing printed-circuit grade copper foil capable of meeting the presently required high quality standards and which can be produced in a satisfactory cost-effective manner.
An object of the present invention is to overcome the draw-backs in prior coatings applied to copper foil and the methods of forming such coatings by providing an improved method of forming a resinous electrocoating on a copper foil. Another object is such a method providing a coating which improves the shelf life of the foil and also makes the foil easier to handle.
A further object of the present invention is a method of forming a resinous coating on copper foil for printed circuit applications which coating has a high dielectric-breakdown voltage, is free of pin-holes, has a high degree of purity and insignificant ion movement or electron tunneling when the thickness is greater than 100.ANG.. Still, a further object is a method of forming a very thin protective resinous coating on such a copper foil which prevents both surface and contact corrosion, as well as preventing contact of zinc alloys with the copper face side of the foil in roll form.
Yet, another object of the present invention is an economic and cost-effective method for coating copper foil which enables the production of a high quality copper foil suitable for use in printed circuit application.
Additional objects and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the present invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.